1. Field of the Invention
This invention relates generally to a system and method for setting the slip in a torque converter of a vehicle and, more particularly, to a system and method for providing a minimum torque converter slip in response to changes in engine speed and transmission gears so as to minimize vehicle driveline vibrations and provide good fuel economy.
2. Discussion of the Related Art
Internal combustion engine vehicles that employ automatic transmissions typically include a torque converter positioned between the engine and the transmission of the vehicle. A torque converter is a fluid coupling device typically including an impeller coupled to an output shaft of the engine and a turbine coupled to the input shaft of the transmission. The torque converter uses hydraulic fluid to transfer rotational energy from the impeller to the turbine. Thus, the torque converter can disengage the engine crank shaft from the transmission input shaft during vehicle idling conditions to enable the vehicle to stop and/or to shift gears.
The rotational speed of the impeller relative to the turbine in the torque converter is typically different so that there is a converter slip therebetween. Because large slips between the engine output and the transmission input significantly affect the fuel economy of the vehicle, some vehicles employ a torque converter clutch (TCC) for controlling or reducing the slip between the engine and the transmission. The TCC can also mechanically lock the impeller at the output of the engine to the turbine at the input of the transmission so that the engine and transmission rotate at the same speed. Locking the impeller to the turbine is generally only used in limited circumstances because of various implications.
Thus, a TCC generally has three modes. A fully locked mode as just described, a fully released mode and a controlled slip mode. When the TCC is fully released, the slip between the impeller and the turbine of the torque converter is only controlled by the hydraulic fluid therebetween. In the slip mode, the TCC controls the pressure of hydraulic fluid in the torque converter so that the slip between the torque converter impeller and the turbine can be set so that is does not exceed a predetermined slip.
Various engine torque perturbations, engine pulses and other engine noises, generally in the range of 30-300 Hz, can be passed through the torque converter from the engine to the transmission and onto the vehicle driveline, which are felt by the vehicle occupants as shaking or vibrations of the vehicle. Typically, these engine pulses and perturbations are more easily passed through the torque converter as the amount of slip between the engine and the transmission is reduced. Thus, for those times when the TCC is locked or has set the slip in the torque converter between the engine and the transmission to be very low, such engine vibrations are typically passed through to the vehicle drive-train. These types of engine disturbances and noise vary depending on the engine speed and the transmission gear.